Binary refrigeration apparatuses of this type have hitherto been used, with a refrigerant circuit shown in the diagram of FIG. 2, for example (see Patent Literatures 1 and 2).
In the figure, S1 denotes a high temperature refrigeration cycle, and S2 denotes a low temperature refrigeration cycle. The discharge-side pipe 2 of a compressor 1 that constitutes the high temperature refrigeration cycle S1 is connected to an auxiliary condenser 3, which is connected to a cascade condenser 11, successively via an oil cooler 4 of the compressor 1, an auxiliary condenser 5, an oil cooler 7 of a compressor 6 that constitutes the low temperature refrigeration cycle S2, a condenser 8, a dryer 9, and a capillary tube 10 in a heat exchanger 30; and further, via a fluid collector 12 and the heat exchanger 30 eventually to the intake-side pipe 13 of the compressor 1. A cooling fan 14 is provided for cooling the respective condensers 3, 5, and 8.
The discharge-side pipe 15 of the compressor 6 in the low temperature refrigeration cycle S2 is connected to an oil separator 16, where a compressor oil is separated to be returned to the compressor 6 via a return pipe 17. The cycle is configured such that the refrigerant flows into the pipe 18 to exchange heat with an intake-side heat exchanger 19, after which the refrigerant passes through the pipe 20 in the cascade condenser 11 to condense, flows through a dryer 21, a capillary tube 22, and into an evaporator 24 from an inlet pipe 23, and then exits from an outlet pipe 25 to return to the compressor 6 through the intake-side heat exchanger 19 and through the intake-side pipe 26 of the compressor 6.
An expansion tank 27 is connected to the intake-side pipe 26 via a capillary tube 28.
CFC (chlorofluorocarbon) and HCFC (hydrochloro fluorocarbon) that were conventionally used as refrigerants in refrigeration apparatuses have been regulated in view of the problem of ozone layer depletion in recent years, and now HFC (hydrofluorocarbon) is being used as a refrigerant instead of these.
With CFC or HCFC used as a refrigerant, hydrocarbon oils such as mineral oil and alkylbenzene have been used preferably as a refrigerator oil. With a different refrigerant, however, the refrigerator oil used therewith may exhibit unpredictable behaviors in terms of compatibility with the refrigerant, lubricity, viscosity of solution in the refrigerant, thermal and chemical stability, and so on, and therefore a new type of refrigerator oil needs to be developed for each refrigerant. For example, ester-based refrigerator oils (see Patent Literature 3) and carbonic acid ester-based refrigerator oils (see Patent Literature 4) to be used with HFC refrigerants have been developed. Among these refrigerator oils, esters are widely used in applications such as refrigerators and air-conditioners.
Thus, a binary refrigeration apparatus configured by connecting a high temperature refrigeration cycle S1 and a low temperature refrigeration cycle S2 via a cascade condenser 11 has been used, wherein the high temperature refrigeration cycle S1 is filled with HFC refrigerants (R-407D, R404A) and an alkylbenzene oil or an ester oil as a refrigerator oil, while the low temperature refrigeration cycle S2 is filled with HFC refrigerants (R-508A, B) and an alkylbenzene oil or an ester oil as a refrigerator oil.
Table 1 shows the refrigerant temperatures at points A to D in the high temperature refrigeration cycle S1 and the refrigerant temperatures at points a to d in the low temperature refrigeration cycle S2 in environments with room temperatures of 30° C. and 5° C., respectively.
TABLE 1REFRIGERANTHIGH TEMPERATURE CYCLE: R404ALOW TEMPERATURE CYCLE: R508BTEMPERATURES OF VARIOUS PARTSHIGH ROOMLOW ROOMTEMPERATURETEMPERATUREENVIRONMENT OF 30° C.ENVIRONMENT OF 6° C.A71° C.38° C.B34° C. 7° C.C−38° C. −49° C. D15° C.−1° C.a67° C.40° C.b−90° C. −96° C. c−89° C. −98° C. d18° C. 4° C.
These HFC refrigerants, however, are also beginning to be regulated because of their high Global Warming Potential (GWP), although their Ozone Depletion Potential (ODP) is zero. Thus, development of a refrigerant that can substitute these HFCs is an urgent issue.
Since a refrigerator oil for lubricating refrigerant compressors usually circulates with the refrigerant in a refrigerant circulation cycle of a refrigeration apparatus, such a refrigerator oil is required to have compatibility with the refrigerant. However, with the use of hydrocarbon oils such as a mineral oil or an alkylbenzene or an ester oil, which are conventionally used for HFC refrigerants, problems would occur in particular with alkylbenzene, for example, which does not provide sufficient compatibility between the refrigerant and the refrigerator oil, such as lubrication failure due to a reduction in the amount of refrigerator oil inside the refrigerant compressor, and blockage of an expansion mechanism such as capillaries, as a result of stagnation of the refrigerator oil in the cycle after discharged from the refrigerant compressor. With an ester oil, while good compatibility with HFC refrigerants is achieved, there was a problem of performance deterioration due to hydrolysis during use.
Another problem was that, one day after the operation of the binary refrigeration apparatus was stopped, for example, when the compressor 1 of the high temperature refrigeration cycle S1 is started up to operate the apparatus, and, upon the refrigerant temperature at the exit of the cascade condenser 11 reaching a predetermined temperature (of, for example, −34° C.), when an attempt is made to start the compressor 6 of the low temperature refrigeration cycle S2, the compressor 6 could not be started up, because the pressure is high in the low temperature refrigeration cycle that was stopped, or even if the compressor 6 could be started, the pressure of the compressor 6 would soon exceed a level determined by safety standards so that a safety device was activated and the compressor was stopped.
Therefore, it was common practice to introduce part of the refrigerant from the intake-side pipe 26 through the capillary tube 28 to the expansion tank 27 to lower the pressure inside the low temperature refrigeration cycle S2, before starting up the compressor 6. The refrigerant introduced to the expansion tank 27 is returned little by little back to the low temperature refrigeration cycle S2 to be used, through the capillary tube 28 after the start of operation of the compressor 6.